NOAA 49RF will fly a Saharan Air Layer Experiment through an African
easterly wave (AEW) and two SAL airmasses as part of IFEX. The G-IV will
leave Barbados at 1450 UTC and will recover back at Barbados at 2200 UTC.
The flight track will take the G-IV east through an area of 'older'
Saharan Air Layer (SAL) air, an AEW axis at ~47°W, and finally through a
'newer' SAL outbreak. These areas and the moist tropical air south of
these features will be sampled on the return east-west leg. The flight
track and 24 dropwindsonde points are shown in Fig. 1.

a) Synoptic Situation
A large SAL outbreak moved off the African continent on 20 September and
by the next day, had pushed as far west as ~30°W (Fig. 2a, SAL 1). An
AEW closely followed this SAL outbreak, emerging from the coast of North
Africa on 22 September (Fig. 2b, AEW 1). As AEW 1 moved across the basin
over the next several days, its northwest heading brought it into the SAL
and it struggled to intensify (Fig. 2c & d). Early on 26 September, it
was determined that the series of SAL outbreaks and the embedded AEW
presented a good case for SALEX and the G-IV was deployed to Barbados.

On the morning of the mission, the AEW axis was located at ~47°W) and the
Meteosat-8 SAL imagery indicated that the environment around the wave was
fairly moist in the lower to middle levels (Fig. 2d). Although there was
some convection associated with this AEW, there were no areas of
concentrated deep convection. However, 850 hPa vorticity analyses from
UW-CIMSS did show that there was a broad area of relative vorticity
associated with this AEW from ~10-20°N and ~40-50°W (Fig. 3).

Fig. 3: 850 hPa relative vorticity analysis for 1500 UTC 27 September
2005. The area of vorticity from ~10-20°N and ~40-50°W is associated
with the AEW (AEW 1) that was investigated during the SALEX mission.
Analysis courtesy of UW-CIMSS.

b) Mission Specifics
The flight pattern was designed to investigate several specific areas (Fig. 4):

An area of SAL air (SAL 1) out ahead (west) of the AEW of interest (AEW 1);

The moist environment associated with the AEW (
AEW 1);

An area of SAL air (SAL 2) behind (east of) the AEW of interest (AEW 1);

A region of convection along the southwestern edge of the second SAL
outbreak (SAL 2). This convective area does not appear to have been
linked to a propagating AEW and was more likely being enhanced by uplift
along the leading edge of SAL 2. TD 19 formed in this area a few days
later (30 September).

Fig. 4: Meteosat-8 SAL tracking imagery for 1200 UTC 27 September 2005.
Red to yellow shading indicates dry, dusty air (associated with the SAL)
in the lower to middle levels of the atmosphere (~600-925 hPa). Two
distinct areas of dry SAL air (SAL 1 & 2), one area of dry polar air
(Polar 1), and an African easterly wave (AEW 1) are indicated in the
imagery. The G-IV flight track and dropsonde points are overlaid on the
image. The AEW axis was located at ~47°W at this time. Imagery courtesy
of UW-CIMSS.

Takeoff was at 1450 UTC from Barbados. The flight plan called an optimal
flight level of 41,000-45,000 ft and required that all dropwindsondes be
transmitted in real-time, so that the humidity data from the sondes could
be included in parallel runs of the NOAA GFS model. The initial flight
pattern involved a short northeast leg from Barbados to the IP at ~14°N 58°W.
The initial drops targeted the leading area of dry SAL air (SAL 1) ahead
of AEW 1 (drops 1-5) and the moisture gradients along its boundaries
(drops 5-7). The next several drops sampled the moist environment of the
AEW (drops 6-11) and the transitional environment between it and the
next SAL outbreak [(SAL 2), drops 11-13]. Drops 13-17 sampled SAL 2 and
the northern edge of the convective area that stretched along ~10°N along
the southwest leading edge of SAL 2. The remainder of the drops (drops
18-24) sampled the moist tropical environment south of SAL 1, AEW 1, and
SAL 2. This last series of drops showed that although the lower to
middle troposphere was quite moist along this transect, the middle to
upper troposphere (~400 hPa–flight level) was very dry. The environment
of SAL 2 was noted to be extremely dry and dusty, with strong temperature
inversions capping the top and bottom layers of the SAL. Dropwindsondes
in SAL 2 (drops 13-17) indicated the following:

RH values of ~0-30% in the typical SAL layer of ~500-850 hPa

The presence of dry air that was also present in the ~200-500 hPa layer

Strong temperature inversions (~1-3oC) in these soundings. These
inversions seemed to correlate with the base and top of the SAL, with a
third inversion often seen at ~300-450 hPa. This third, higher altitude
inversion may have been associated with subsidence above the SAL layer.

Photos taken from the G-IV just after drop #15 (~1820 UTC, ~14°N 35°W)
indicated that the atmosphere was extremely dusty below the flight level.
This could be seen as a dense layer of white haze at roughly the level of
the low cumulus clouds that were present. The dust was much easier to
see visually when one looked in the direction of the sun, suggesting that
the SAL is mineral dust much more effectively forward scatters sunlight
than it backscatters sunlight.

Fig. 5: Photos taken from the NOAA G-IV jet from ~45,000 ft. Both photos
were taken at ~1820 UTC at ~14°N 35°W just south of drop point #15 during
the SALEX mission and are looking west on a north-south leg. Saharan
mineral dust can be seen as a milky white

Problems :

There were no major problems related to this flight. The vintage 2000
GPS dropsondes that were used did, however, have a fairly high failure
rate (~30% or 10 failed dropwindsondes out of 33 deployed).